: A critical link between the activation of the ras proto-oncogene and transcriptional activation is the transactivation of c-Jun and AP-1 transcription factors. The applicant has created a null mutation of c-jun in the mouse. Primary cells derived from such embryos growth arrest immediately in culture. Growth arrest can be overcome by co-transformation with large T antigen and activated Ras; however, these cells, unlike wild type cells transformed by the same agents, remain non-tumorigenic. These data indicate a critical role of c-jun in Ras induced tumor formation. The applicant proposes to further elucidate this role using a combined biochemical and genetic approach. First, he will perform targeted replacement of c-jun with a S63/S73 mutant, to assess role of JNK-mediated phosphorylation on c-jun function in vivo. Similar approaches will be used to determine the role of and the requirement for the delta region, which is thought to play a role both in JNK recruitment and in ubiquitination/destabilization of c-jun. The possible redundancy of different c-jun genes will be assessed by studies in which c-jun is replaced by jun B or jun D. In addition, the applicant will assess the role of junD directly by performing a junD knockout. Finally, the applicant will use a novel in vivo assay of tumorigenic establishment and screen for candidate target genes involved in ras/AP-1 mediate tumorigenesis.